There are various systems for synchronizing or phase locking a local clock to an incoming data stream in order to be able to properly identify one bit of data from another. Common in such systems are phase locked loops which operate to initially adjust the phasing of a local oscillator to the leading or trailing edge of a bit interval in the incoming data with great accuracy. A phase locked loop is then closed around the local oscillator with a bit detector that operates to control the local oscillator phasing continuously to maintain a high level of synchronization between the local clock and the incoming data.
Such systems are capable of attaining and maintaining very accurate phase control of the local oscillator over extended periods of time. One drawback of such systems, or indeed of other systems that are designed to maintain long term oscillator lock, is the length of time needed for the synchronization system to initially achieve phase locking. This time can be microseconds in length, for example 6.4 microseconds is required for synchronization in ETHERNET running at 10 MHz. Such time intervals may not be objectionable where normal lengths of data packets are involved.
A data packet is the group of digital signals exchanged between stations on a telecommunications network before the stations relinquish their connection to permit another set of stations to communicate over the same channel. Long packets are desirable where data is being exchanged and there is no need for real time information exchange.
Where voice signals are being exchanged on a digital signal path, however, there is a need to maintain both real time communication and short data packets to insure that the voice retains its natural sound and the exchange between parties is conversational. In this case, the entire data packet may typically be about 25 microseconds in length. Here a synchronization interval of several microseconds would create an intolerable inefficiency. A 6.4 microsecond synchronization interval would waste 25 percent of the data time.
Before synchronization can occur, the data signal received over a network must be processed in level and amplitude in order to permit binary information to be extracted. This adjustment needs to be accomplished in a very short period of time.